1. Field of the Invention
The present invention relates to frequency-voltage converters which make it generally possible to convert recurrent signals into an electric magnitude proportional to their recurrence frequency, the electric magnitude being very often a voltage level.
2. Description of the Prior Art
When confronted with a recurrent phenomenon and it is desired to measure the recurrence frequency of the phenomenon, it is usual to use a frequency-voltage converter having a monostable multivibrator followed by an RC type filter. Each occurence of the phenomenon triggers the multivibrator which delivers a calibrated pulse. The signal obtained is applied to the input of the filter which delivers a voltage proportional to the number of input pulses per unit of time, that is to say proportional to the frequency it is desired to measure. The drawback of such a device is that the indication of the frequency can only be obtained after a certain number of pulses, which means that this frequency cannot be known instantaneously.
This has in fact not been a real disadvantage as long as it was not required to construct frequency-voltage converters which give a non instantaneous indication of the frequency of the recurrent event and this was the case in the field of automobile electronics, where it was only desired to construct an apparatus for measuring the speed of the engine so as to cause it to appear on the speedometer of the dashboard of the motor vehicle. It was not necessary for the measurement to be instantaneous.
The use of such converters changed however with the appearance of electronic ignition systems. In fact, these systems use as one of their input magnitudes the instantaneous value of the speed of the engine. It is important to provide an electric signal making it possible to identify certain angles of rotation of the engine. The conventional way of doing this is to charge or discharge a DC capacitor, from a voltage initialized at each period. Then, so that the instantaneous voltage represents the angle of rotation of the engine, the discharge current must be proportional to the speed, but during acceleration or deceleration of the engine, measurement of the speed is erroneous, the angle information is falsified and the whole ignition cycle is compromised.
Such a system cannot give good results for it does not have available, at each period, the true value of the instantaneous frequency.
The present invention overcomes this drawback by providing a frequency-voltage converter which makes it possible to known the instantaneous value of the recurrence frequency of a phenomenon, that is to say the value of the frequency for a recurrence with respect to the preceding recurrence, that is to say again the inverse of the last period.